The present invention pertains to an integrated flow-control valve of compact design, adapted for precisely controlling the flow of gases in semiconductor processing applications.
In applications such as semiconductor processing, valves for fluid flow control must exhibit several particular capabilities. First, they must provide precise control of fluid flow volumes, including virtually absolute shutoff to as little as about 10xe2x88x927 sccm, as well as virtually zero outleakage to the environment of often extremely toxic and corrosive process fluids. They must also maintain the required high purity of the fluids, contributing no appreciable amount of particulates, which are typically generated by wearing parts within the wetted portion of the valve. They must possess good resistance to corrosive properties of the fluids. Due to the toxicity of a number of the fluids transported, very high reliability and long service life (avoidance of the need to shut down and change out parts) are of great importance. Less important, but still highly desirable, are a compact design which requires limited space, and a reasonable cost.
For semiconductor applications, a valve body where all of the fluid-wetted parts are fabricated from a highly corrosion-resistant alloy has distinct advantages. Process control valves frequently employ corrosion-resistant plastic or elastomeric valve seats. Metal valve seats provide advantages in terms of minimizing valve seat maintenance and maintaining fluid purity; however, metal valve seats require high seating forces, compared to polymeric seats, in order to reliably provide a tight shut-off. As a result, all valves with metal valve seats are typically larger in size and cost significantly more than valves with polymeric seats.
One example of a valve having metal-to-metal seating for controlling the flow of a gas employs a flexible metal diaphragm mounted in the valve so the diaphragm can be moved into and out of sealing contact with the metal seat to close and open a gas passage, respectively. The valve seat has a rounded metal sealing projection with a relatively small cross-sectional radius around the seating section extending about the gas flow passage. The flexible metal diaphragm is moved into and out of sealing contact with the metal sealing projection of the seat by an actuator which employs a metal backing member which forcefully contacts the diaphragm during narrowing or closing of the gas flow passage. The metal sealing projection of the seat is formed of a relatively soft metal and each of the flexible metal diaphragm and the metal backing member of the actuator is formed of relatively hard metal. The metal backing member has a contour which is convexly rounded at a surface which contacts the diaphragm on the side of the diaphragm opposite the sealing projection of the metal seat. For additional information about this all-metal valve, one skilled in the art should refer to U.S. Pat. No. 5,755,428, of Louis Ollivier, issued May 26, 1988.
As described above, the potential problems of process fluid outleakage and/or process fluid attack on valve mechanicals may be addressed using a diaphragm valve (among other closing techniques). A thin diaphragm or membrane may be used to form a leak-tight seal between the portion of the valve interior through which process fluid passes (the xe2x80x9cwetted sectionxe2x80x9d of the valve), and the portion of the valve body containing the moving mechanisms that open and close the valve (the xe2x80x9cdrive sectionxe2x80x9d of the valve). The diaphragm is pressed against the valve seat by a moving stem or the like to effect closure of the valve. When the valve seat is metal, a particularly high seating force is required, compared with polymeric valve seats. The valve is typically operated in a normally-closed position, to provide a xe2x80x9cfail safexe2x80x9d condition in the event of a loss of motive power (electric or pneumatic) to the actuator. The normally-closed condition requires application of a force against the diaphragm, with the actuator designed to overcome the force in order to open the valve. When the actuator design incorporates a spring (or springs) capable of applying the large force required for a metal valve seat, the spring is typically large, on the order of 3 cm to 10 cm tall, and the valve itself is expensive, often costing around 5-6 times the price of a comparable capacity plastic-seated valve.
It would be highly desirable to have a highly corrosion-resistant fluid on/off valve, where all of the fluid-wetted parts are metal; where the valve is compact in design; and, where the valve price is competitive with valves having fluid-wetting parts fabricated from materials other than metal (or where the valve price is competitive in terms of long-term operational costs).
The present invention pertains to the design of a compact integrated fluid on/off valve, adapted for use in controlling process fluids in semiconductor processing operations. The fluid-wetted surfaces of the valve are fabricated from a corrosion-resistant metal or metal alloy, including a metallic diaphragm separating the wetted section of the valve from the non-wetted drive section of the valve. In the wetted section, process fluids enter through one or more entrance ports. The exit port from the wetted section comprises an annular metallic valve seat. The valve seat is formed as, or upon, an inner lip of the exit port. When the valve is closed, the fluid flow is interrupted by a section of the diaphragm being pressed tightly against the valve seat.
The metal valve seat is advantageously a dynamic seat which deforms elastically when pressed upon by the diaphragm and its backing disk, so that the seat recovers each time the valve is closed and reopened, rather than being permanently deformed by the valve""s closure.
In the valve""s wetted section, metal-to-metal bonding can advantageously be accomplished by diffusion bonding. Diffusion bonding is a direct bonding process which provides smooth, strong bonds that do not absorb or release process fluids, and do not contribute impurities to the process fluids, as a welded joint might do. It is important that no adhesive (or adhesive residue) be present on the wetted flow path. The diffusion bonding permits the formation of complex shapes without costly machining.
In order for diffusion bonding to be effective, the metal surfaces that are to be bonded must have a surface roughness within the range of about 0.5 Ra to about 30 Ra prior to diffusion bonding. Typically, the metal surfaces have a surface roughness within the range of about 0.5 Ra to about 10 Ra; more typically, within the range of about 1.5 Ra to about 5 Ra. We have found that diffusion bonding works quite well when the metal surfaces have a surface roughness within the range of about 1.5 Ra to about 3.0 Ra.
In many cases, the metal surfaces will need to be pretreated by chemical etching, or a combination of mechanical planarization and chemical etching, to have the desired surface roughness prior to diffusion bonding. For example, stainless steel can be chemically etched using ferric chloride according to standard methodology known in the art. A process for electrochemical etching of difficult to etch materials such as HASTELOY is described in U.S. Pat. No. 6,221,235, issued Apr. 24, 2001, to Gebhart. Certain materials may require mechanical planarization to smooth down the surface prior to the performance of a chemical etching process to obtain a surface roughness within the desired range. Mechanical planarization of metal surfaces can be performed according to techniques known in the art.
In some instances, the metal surfaces may be manufactured with the desired surface roughness, and may need no chemical or mechanical pretreatment prior to diffusion bonding. For example, the ASTM standard for surface roughness for stainless steel sheets (as rolled) is 0.5 Ra to 4.0 Ra (ASTM 480 BA).
In the valve""s non-wetted drive section, a sliding cylinder moves up and down, pressing a lower horizontal member, which typically includes a convex contacting surface, against a diaphragm, which is in turn pressed against the valve seat to close the valve. The diaphragm is permitted to move away from the valve seat to open the valve. The sliding cylinder has an upper horizontal member connected to a smaller, lower horizontal member by a vertical member. The sliding cylinder may be of single piece construction. The valve is maintained in a normally-closed position by a spring force applied at the top of the upper horizontal member of the sliding cylinder. The spring drives the lower horizontal member of the sliding cylinder, which includes a convex surface (acting as a backing disk), against the diaphragm. The sliding cylinder has a gas-tight seal around the perimeter of both the upper horizontal member and the lower horizontal member. A typical gas-tight seal is a polymeric xe2x80x9cOxe2x80x9d-ring. The valve is opened by pneumatic force from a pressurizing gas which is applied in a space between the upper horizontal member and the lower horizontal member of the sliding cylinder. When the pressurizing gas is applied within the space, the pressurizing gas acts to compress a spring or spring assembly located above the upper horizontal member, permitting the sliding cylinder to rise, and permitting the diaphragm beneath the lower horizontal member to rise above the metallic seat, enabling fluid to flow through the annular opening within the metallic seat.
The use of Belleville springs to provide the closing force permits a much more compact valve actuator than the use of coil springs.
In the drive section of the valve which is not wetted by fluids, metal-to-metal bonding may be advantageously accomplished using high-strength adhesives, which do not require subjecting valve mechanicals to the increased temperatures and pressures involved in performing the diffusion bonding used in the wetted section, and which provide a simpler and cheaper alternative to diffusion bonding.